1. Field of the Invention
This invention relates to a process for producing a non-oriented electromagnetic steel strip that has enhanced magnetic qualities and can retain these qualities in a uniform or regular condition in a product coil.
2. Description of the Related Art
Non-oriented electromagnetic steel strips are well-suited as core material for motors, generators and transformers. To increase energy efficiency, non-oriented electromagnetic steel strips should exhibit excellent magnetic characteristics, i.e. low iron loss and high magnetic flux density.
Magnetic characteristics can be improved by modifying the aggregate structure of the corresponding product steel strip, i.e., by decreasing (111)-oriented crystal grains and by increasing (100)-oriented crystal grains. As is well known in the art, the metallic structure of a hot-rolled steel strip has a major effect on the aggregate structure of the resulting product steel strip. As a result, it has been widely recognized that non-oriented electromagnetic steel strip varies in magnetic qualities depending upon the temperatures at which hot rolling is completed and at which hot-rolled steel strip take-up is conducted. These temperature parameters are closely associated with the metallic structure of a hot-rolled steel strip, which in turn strongly influences the aggregate structure of a product steel strip.
With a view to improving the magnetic quality of electromagnetic steel strip, Japanese Patent Laid-Open (Unexamined) Publication No. 51-74923 utilizes the above-described relationships. This publication discloses a method which calculates a transformation point A.sub.3 from the equation EQU A.sub.3 ={820+30([Si wt %]+3[Al wt %]-6 [C wt %])}.degree. C
and, at the same time, completes finish-hot rolling between a temperature calculated from the equation EQU {750+30 ([Si wt %]+3[Al wt %]-6[C wt %])}.degree. C.
and a temperature calculated from the equation EQU {810+30([Si wt %]+3[Al wt %]-6[C wt %])}.degree. C.
whereby an electromagnetic steel strip is produced having good magnetic quality, low silicon content and minimal thickness irregularity.
However, even when hot rolling is completed within the temperature range proposed by that publication, the resultant product possesses a magnetic flux density (B.sub.40) of 1.72 (Wb/m.sup.2), which is only marginally better than the 1.71 (Wb/m.sup.2) achievable through conventional methods.
To further improve magnetic quality in an electromagnetic steel strip, Japanese Patent Laid-Open (Unexamined) Publication No. 56-38420 computes transformation points Ar.sub.3 and Ar.sub.1 from the equations EQU Ar.sub.3 ={891-90(C %)+50(Si %)-88(Mn %) +190(P %)+380(Al %)}.degree. C.
and EQU Ar.sub.1 ={882-5,750(C %)+58,800(C %)2+50(Si %) -82Mn %)+170(P %)+380(Al %)}.degree. C.,
thereby completing hot rolling at a temperature lower than (Ar.sub.3 +Ar.sub.1)/2 and higher than 750.degree. C. and also performing take-up at a temperature above 680.degree. C. The high take-up temperature, however, results in the formation of thick scales on the hot-rolled steel strip. Removal of the scales requires extensive pickling, thereby sharply increasing production cost.
Motors having integrated circuits (ICs) built therein have become commonplace. Such motors require precise controllability; thus, irregularities between like motors must be kept at an absolute minimum. As a result, a strong demand has arisen for a non-oriented electromagnetic steel strip which not only exhibits excellent magnetic quality, but also uniformly retains its magnetic quality in the product coil.
The foregoing prior art publications do not consider the uniform retention of magnetic quality in the product coil at all. Where a hot-rolled steel strip is taken up at above 680.degree. C. as taught by Japanese Patent Laid-Open (Unexamined) Publication No. 56-38420, a product coil fabricated from the steel strip is cooled such that the outer and inner portions of the strip are exposed to two considerably different temperatures. Consequently, the magnetic qualities of the product coil become irregular throughout the coil, thereby destroying the core uniformity demanded in motors having ICs.